This covers both LSR and HCR silicones as well as thermoplastics on equipment that includes four injection molding machines, three of which equipped for molding LSR. Unipace has eight scientific and technical staff, supported by six student assistants. A planetary dissolver has been added at the end of March, and Giesen said there were plans to soon start using software from Sigmasoft to simulate molding of LSR parts.
Giesen explained that although LSR bonds strongly to nylon and PBT, it requires special grades containing adhesion promotion additives to achieve LSR/PC bonding. Unipace has therefore sought to achieve LSR/PC combinations using standard LSR grades from Dow Corning, Wacker Chemie and Momentive. It has been doing this with UVC-C in-mold treatment that does not involve heat entering the thermoplastic and that also allows for partial bonding where required, by selectively desisting from treatment of some areas.
Giesen mentioned that alternative techniques known to work with LSR/PC include using an adhesive layer and treatments such as Plasamtreat's Openair low-pressure plasma, as well as corona and flame or silica coating treatments. The UV-C treatment can be applied as pre-treatment, with separate 9,000-hour lifetime UV lamps or with such lamps integrated in the linear robotic handling system or by grippers operating in the mold, the latter as shown at Arburg.
Although Unispace uses an EDEGS mold in its LSR/PC work, other equipment differs from that shown at the Arburg open house, namely use of LSR 200 ELA 200 and EMT dosing equipment from respectively German suppliers Reinhardt Technik GmbH in Kierspe and EMT Dosiertechnik (Efficient Metering Technology) in Werdohl.
The entire LSR/PC process, for which Unipace has applied for a patent, involves overall cycle time of 60 seconds, including around five seconds of pre-treatment within the mold area with the robotic handling system and with another five seconds needed to move the molded PC plate from one cavity to the other and 45 seconds for PC and LSR injection and LSR curing stages.
Based on work with LSR from Wacker Chemie and Makrolon brand PC from Covestro, it was found that while LSR bond strength to PC drops slightly after five second UV-C exposure and is at a similar level for LSR/nylon. Giesen showed a chart, however, in which the LSR/PA bond strength rose well above that for LSR/PC at 30 seconds, reaching around 3.25 N/mm. He concluded however that 5-10 second radiation time suffices to obtain good adhesion of LSR to PC and PC/ABS.
Giesen pointed to a particular advantage of the process: its low cost compared with other bonding processes such as plasma treatment. There are also no health risks beyond a 5-10 centimeters distance from the UV-C source, he stated. Adhesion has remain high after 12 months and UV-C treated PC granulate has also been found to enable high adhesion to LSR after it was sent for a trial at an external molding processor, Giesen said.